Increasingly severe automobile regulations in respect of emissions and fuel efficiency are placing increasing demands on both engine manufacturers and lubricant formulators to provide effective solutions to improve fuel economy.
Friction-reducing additives (which are also known as friction modifiers) are important lubricant components in reducing fuel consumption and various such additives are already known in the art.
Friction modifiers can be conveniently divided into two categories, that is to say, metal-containing friction modifiers and ashless (organic) friction modifiers.
Organo-molybdenum compounds are amongst the most common metal-containing friction modifiers. Typical organo-molybdenum compounds include molybdenum dithiocarbamates (MoDTC), molybdenum dithiophosphates (MoDTP), molybdenum amines, molybdenum alcoholates, and molybdenum alcoholamides. WO-A-98/26030, WO-A-99/31113, WO-A-99/47629 and WO-A-99/66013 describe tri-nuclear molybdenum compounds for use in lubricating oil compositions.
However, the trend towards low-ash lubricating oil compositions has resulted in an increased drive to achieve low friction and improved fuel economy using ashless (organic) friction modifiers.
Ashless (organic) friction modifiers typically comprise esters of fatty acids and polyhydric alcohols, fatty acid amides, amines derived from fatty acids and organic dithiocarbamate or dithiophosphate compounds.
Further improvements in lubricant performance characteristics have been achieved through the use of synergistic behaviours of particular combinations of certain lubricant additives.
WO-A-99/50377 describes a lubricating oil composition which is said to have a significant increase in fuel economy due to the use therein of tri-nuclear molybdenum compounds in conjunction with oil soluble dithiocarbamates.
EP-A-1041135 describes the use of succinimide dispersants in conjunction with molybdenum dialkyldithiocarbamates which is said to give improved friction reduction in diesel engines.
U.S. Pat. No.-B1-6,562,765 describes a lubricating oil composition which is said to have a synergy between an oxymolybdenum nitrogen dispersant complex and an oxymolybdenum dithiocarbamate which leads to unexpectedly low friction coefficients.
EP-A-1367116, EP-A-0799883, EP-A-0747464 U.S. Pat. No. 3,933,659 and EP-A-335701 describe lubricating oil compositions comprising various combinations of ashless friction modifiers.
WO-A-92/02602 describes lubricating oil compositions for internal combustion engines which comprise a blend of ashless friction modifiers which are said to have a synergistic effect on fuel economy.
The blend described in WO-A-92/02602 is a combination of (a) an amine/amide friction modifier prepared by reacting one or more acids with one or more polyamines and (b) an ester/alcohol friction modifier prepared by reacting one or more acids with one or more polyols.
U.S. Pat. No. 5,286,394 describes a friction-reducing lubricating oil composition and a method for reducing the fuel consumption of an internal combustion engine.
The lubricating oil composition described therein comprises a major amount of an oil having lubricating viscosity and a minor amount of a friction-modifying, polar and surface active organic compound selected from a long list of compounds including mono- and higher esters of polyols and aliphatic amides. Glycerol monooleate and oleamide (i.e. oleylamide) are mentioned as examples of such compounds.
However, current strategies with regard to friction reduction for fuel economy oils are not sufficient to meet ever increasing fuel economy targets set by Original Equipment Manufacturers (OEMs).
For example, molybdenum friction modifiers typically outperform ashless friction modifiers in the boundary regime and there is a need and a challenge to approach similar levels of friction modification using solely ashless friction modifiers.